How to identify, treat and stop Japanese knotweed spreading in your garden


Due to the longer days and more sunlight, the pace of growth in the garden is frenetic in May. This is clear to see as the fruit is just starting to appear, the roses are coming out, and your lawns now need your constant attention.

Amongst all this activity, however, Japanese knotweed will also be running amok!

Introduced to the UK in 1850 through the Botanic Gardens at Kew, native Knotweed has quickly grown into one of the most invasive plant species in the UK. This is due to the fact that it was introduced to the UK without the insect (called a ‘psyllid’) that keeps its growth under control, though recent psyllid release programmes have been introduced in England and Wales in a bid to ease its spread.

With that in mind, what should you know about the spread, identification, and treatment of knotweed on your property?

The spread of Japanese knotweed

Since its original introduction to the UK, Japanese knotweed has rapidly spread across the country. In fact, a 2010 survey by Williams et al. estimated that almost 10% of rivers in the country were invested, while DEFRA (the Department for Environment, Food and Rural Affairs) also theorised that it will cost £1.6bn to eradicate the spread of the plant specifies.

But what has allowed Japanese knotweed to become such a household name is the threat it poses to buildings. As it grows vigorously (up to 20cm a day), it can reach a total of four meters in height, while its roots can burrow 3 metres deep and up to seven metres horizontally. Because of this rapid growth, it can cause structural damage to by creeping into bricks, cavity walls and other structural components of a building.

The law around Japanese knotweed

Firstly, it’s important to know that it isn’t illegal to have Japanese knotweed present on your grounds. However, as the GOV.UK website explains, you should stop what is already present on your land from spreading to the wild and causing a nuisance, though it doesn’t need to be completely removed if it’s properly controlled.

Although the Anti-social Behaviour, Crime and Policing Act 2014 doesn’t explicably refer to knotweed, this community protection notice can be served against individuals, businesses and organisations that neglect to prevent it from spreading into the wild.

Japanese knotweed leaves

Japanese knotweed leaves (source)

This is because the great structural damage it causes to properties can have a detrimental effect on communities.

If an individual is unable to prevent the spread of knotweed (and any other contaminated soil or plant material) into the wild, they could be fined up to £5,000 and face up to 2 years in prison.

The identification of Japanese knotweed

Japanese knotweed can be tricky to identify as many shrubs and small trees look similar in appearance. Despite having distinctive shovel/heart shaped leaves that are flat and green in colour, Dogwood and Lilac are often confused for knotweed as they have a similar leaf shape.

As a key feature of knotweed is its panicles of small cream flowers, the plant is easier to identify in the summer months as these flowers will have bloomed by late August. In the summer, the leaves are also more easily identifiable as they will grow in a distinctive ‘zigzag’ pattern.

Japanese knotweed in summer

Japanese knotweed in summer (source)

Nonetheless, there are a few identifying features you can look out for in the other months of the year. For example, when knotweed begins to sprout in Spring, its small shoots will look similar to asparagus, though they will have a red/purple tinge. However, when it grows taller and the leaves start to sprout, Japanese knotweed will look more like bamboo in appearance.

In autumn, the leaves will continue to have the same distinctive zigzag pattern as they do in summer, and its small cream flowers will still be present. As well as having a lot of foliage at this point, the stems will change from a reddish-brown to darker shade of brown, while the shovel/heart shaped leaves will begin to wilt and turn yellow in colour.

As knotweed becomes dormant in the winter months, its dark brown canes will lose their leaves, start decomposing, and many will become hollow. The hollow canes will also begin to collapse and become entwined with each other.

You’re also likely to find older canes underneath the most recent growth of Japanese knotweed (which will be in various stages of decomposition), as well as next season’s shoots.

Japanese knotweed in winter

Japanese knotweed in winter (source)

The treatment of Japanese knotweed

Although it grows rapidly, you can treat and prevent the spread of Japanese knotweed with a chemical called Glyphosate or by digging out the growth.

To treat knotweed in this way, apply the Glyphosate-based weedkiller directly to the foliage. As the RHS website explains, this ensures it passes within the plant to the underground parts. It’s also advised that you cut away old stems during the previous winter, as this will provide better access to the foliage. However, the most effective time to treat Japanese knotweed is when it’s at the flowering stage. This will typically be during the late summer.

When treating Japanese knotweed, it’s important to be aware that it usually takes between three to four seasons to completely eradicate it using Glyphosate products.

If you decide to dig out the knotweed instead, you should bear in mind that regrowth is likely to occur. If you do choose this method, you’ll need to remove the woody rhizome-clumps from which the roots sprout from.

Alternatively, you can use a commercial Japanese knotweed treatment and stem removal service from a specialist company, such as PHS Greenleaf. This is option is often required to allow for your insurance to be valid as there are regulations around the disposal of Japanese knotweed that need to be adhered to. As knotweed is classed as ‘controlled waste’ under the Environmental Protection Act 1990, it needs to be disposed of at licensed landfill sites.

By using a commercial knotweed removal service, you can ensure it’s correctly disposed of, as well as greatly speeding up the removal process!